This invention relates to a polyester composition having improved transparency and heat resistance. More particularly, it relates to a polyester composition comprising a polyalkylene terephthalate and a polycarbonate in which the glass transition temperature as measured by a differential scanning calorimeter (DSC) exhibits a single transition and from which a molded article having improved transparency and heat resistance and increased mechanical strength can be produced. It also pertains to a transparent molded polyester laminate having improved heat resistance as well as improved mechanical properties including impact resistance. More particularly, it pertains to a transparent molded polyester laminate comprising a layer formed of polyalkylene terephthalate and another layer of a polyester composition of polyalkylene terephthalate and polycarbonate, having improved heat resistance as well as improved mechanical properties including impact resistance.
Resin compositions comprising a polyester such as polyethylene terephthalate and a polycarbonate are well known in the art from the literature and patent publications including Japanese Patent Application Kokai Nos. 52-51445, 53-51248, 54-18375, 55-145751, and 58-500614, Japanese Patent Publication Nos. 36-14035, 57-16137, and 58-18391, J. W. Barlow et al, Journal of Applied Polymer Science, Vol. 23, 85-99 (1979), and Hi Hui Wang et al, Makromoleculre Chemie Rapid Communications, Vol. 7, 255-259 (1986). In these publications, attempts were made to produce a highly transparent composition from polyester and polycarbonate. Japanese Patent Application Kokai Nos. 52-51445 and 53-51248 and Japanese Patent Publication No. 58-18391, however, describe that only compositions comprising a major proportion of polyester or polycarbonate are highly transparent. In either of these publications, it is described that no composition with high transparency is available when the proportions of polyester and polycarbonate blended approach to be equal. The polyester base compositions described in these publications have excellent chemical resistance despite transparency, but are poor in such properties as heat resistance and impact resistance. These compositions also suffer from the likelihood of sink marks and warping occurring on the appearance of parts molded therefrom. The polycarbonate base compositions have excellent heat resistance despite transparency, but are poor in such properties as chemical resistance, stress crack resistance, and melt flow. We have made follow-up tests to find that when a polyester and a polycarbonate are blended in a proportion of from 80:20 to 20:80 by weight according to the methods proposed in the above-cited publications, the resulting resin compositions do not fulfil a desired profile of melt flow, transparency, heat resistance, mechanical strength and chemical resistance.
Hi Hui Wang et al, Makromoleculre Chemie Rapid Communications, Vol. 7, 255-259 (1986) incorporated herein by reference report that approximately equal proportions of polyester and polycarbonate give a transparent resin composition having a single glass transition temperature. The resin compositions obtained according to the teaching of this literature, however, cannot be used as the molding material from which food containers and medical equipment parts are molded because they have a low molecular weight and hence, a low mechanical strength as typified by impact resistance.
Glass is widely used as containers for seasonings, oil, juice, carbonated beverage, beer, wine, cosmetics, and detergents. Since glass containers are relatively expensive to manufacture, recycling is recommended to recover empty containers for reuse. Glass containers require a more cost for transportation because of heavy weight and suffer from the drawbacks that they are breakable and need careful handling.
Plastic containers which are free of the above-mentioned drawbacks appeared as a substitute for glass containers. Recent conversion from glass to plastic containers takes place very rapidly. The material of which containers are made is selected from a variety of plastic materials depending on the type of filling contents and the intended application. Among various plastic materials, polyethylene terephthalate is utilized as the stock material for molding containers for juice, refreshments, carbonated beverage, seasonings, detergent, and cosmetics because of heat resistance, transparency, and gas barrier. Among these applications, molded hollow containers which are to be filled with juice, refreshments and carbonated beverage are required to come up with a high speed filling process. A heat-resistant resin composition which withstands high-temperature filling is required because concurrent sterilization is desired. These molded hollow containers must be transparent so that the filling is visible from the outside. Although polyethylene terephthalate is a plastic material which is excellent in the necessary physical properties, a transparent polyethylene terephthalate having a sufficient heat resistance to comply with the above-mentioned high-temperature filling has never been available.
Several methods are known in the prior art for molding heat-resistant hollow containers from polyethylene terephthalate, including lamination of a heat-resistant resin such as polyarylate (see Plastics, Vol. 36, No. 9, 121 (1985)), molding followed by heat setting (see Japanese Patent Publication No. 59-3301, Japanese Patent Application Kokai No. 55-12031, 56-75833 and 56-13142), and treatment of molded containers with a solvent to improve crystallinity (see Japanese Patent Publication No. 59-15807). These methods impart heat resistance to the polyethylene terephthalate material which has poor heat resistance by nature by using special molding means or by applying a treatment after molding. Neither of the molded hollow containers produced by these methods were fully transparent or satisfactorily resistant to the heat applied during high-temperature filling of juice. In the field of application of molded hollow polyethylene terephthalate containers where high-temperature filling is required, there is a strong need for a hollow container-molding material which has a sufficient heat resistance and mechanical properties to comply with high-temperature filling, while maintaining the transparency of polyethylene terephthalate per se.